1. Field of the Invention
The present invention relates to a multipoint focus detecting apparatus for an optical instrument such as an SLR camera.
2. Description of the Prior Art
In recent years, SLR cameras provided with a multipoint focus detecting unit for determining a focus state (defocus) at each of a plurality of focus detection zones (focusing points) have been developed. In the optical system of a conventional multipoint focus detecting unit, other than the central focus detection zone arranged over the optical axis, one or more off-center focus detection zones are arranged away from the optical axis, and light beams of an object image which are passed through the center and off-center optical axes are respectively deflected by corresponding mirrors to be incident on corresponding light receiving elements arranged in a horizontal line. For instance, a light beam of an object image which is passed through the horizontally-elongated central focus detection zone is deflected once by a mirror to be focused on a corresponding horizontally-elongated light receiving element, while another light beam of the object image which is passed through a vertically-elongated off-center focus detection zone is deflected twice by two mirrors to be focused on a corresponding horizontally-elongated light receiving element.
According to such arrangement, the light beams which are respectively passed through the center and off-center focus detection zones need to be independently deflected and guided to a corresponding light receiving element. Accordingly, it is necessary to increase the number of the mirrors or the size thereof as the number of the off-center focus detection zones increases. However, in the case where large mirrors are used, there is a possibility of the mirrors intercepting an optical path in the multipoint focus detecting unit. In the case of using a large number of mirrors, it is difficult to arrange the mirrors so as not to intercept each other while avoid intercepting the optical path in the multipoint focus detecting unit. Furthermore, since the light beams, which are respectively passed through the center and off-center focus detection zones, travel so close to each other, there is a possibility of each of the light beams being intercepted by an optical system provided for leading another light beam to the corresponding light receiving element thereof.